A. Background
The development of methods for the sequential degradation of proteins and peptides from the carboxy-terminus has been the objective of several studies. See Ward, C. W., Practical Protein Chemistry--A Handbook (Darbre, A., ed.) (1986) and Rangarajan, M., Protein/Peptide Sequence Analysis: Current (1988). Such a method would complement existing N-terminal degradations based on the Edman chemistry. Edman, P., Acta.Chem.Scand. 4:283-293 (1950). The most widely studied method and probably the most attractive because of its similarity to the Edman degradation has been the conversion of amino acids into thiohydantoins. This reaction, originally observed by Johnson and Nicolet, J.Am.Chem.Soc. 33:1973-1978 (1911), was first applied to the sequential degradation of proteins from the carboxy-terminus by Schlack and Kumpf, Z.Physiol.Chem. 154:125-170 (1926). These authors reacted ammonium thiocyanate, dissolved in acetic acid and acetic anhydride, with N-benzoylated peptides to form carboxyl-terminal 1-acyl-2-thiohydantoins. Exposure to strong base was used to liberate the amino acid thiohydantoin and generate a new carboxyl-terminal amino acid. The main disadvantages of this procedure have been the severity of the conditions required for complete derivatization of the C-terminal amino acid and for the subsequent cleavage of the peptidylthiohydantoin derivative into a new shortened peptide and an amino acid thiohydantoin derivative.
Since this work was published, numerous groups have tried to reduce the severity of the conditions required, particularly in the cleavage of the peptidylthiohydantoin, in order to apply this chemistry to the sequential degradation of proteins from the carboxyl terminal end. Lesser concentrations o sodium hydroxide than originally used by Schlack and Kumpf and of barium hydroxide were found to effectively cleave peptidylthiohydantoins. See Waley, S. G., et al., J.Chem.Soc. 1951:2394-2397 (1951); Kjaer, A., et al., Acta Chem.Scand. 6:448-450 (1952); Turner, R. A., et al., Biochim.Biophys.Acta. 13:553-559 (1954). Other groups used acidic conditions based on the original procedure used by Johnson and Nicolet for the de-acetylation of amino acid thiohydantoins. See Tibbs, J., Nature 168:910 (1951); Baptist, V. H., et al., J.Am.Chem.Soc. 75:1727-1729 (1953). These authors added concentrated hydrochloric acid to the coupling solution to cause cleavage of the peptidylthiohydantoin bond. Unlike hydroxide which was shown to cause breakdown of the thiohydantoin amino acids, hydrochloric acid was shown not to destroy the amino acid thiohydantoins. See Scoffone, E., et al., Ric.Sci. 26:865-871 (1956); Fox, S. W., et al., J.Am.Chem. Soc. 77:3119-3122 (1955); Stark, G. R., Biochem. 7:1796-1807 (1968). Cromwell, L. D., et al., Biochem. 8:4735-4740 (1969) showed that the concentrated hydrochloric acid could be used to cleave the thiohydantoin amino acid at room temperature. The major drawback with this procedure was that when applied to proteins, no more than two or three cycles could be performed.
Yamashita, S., Biochem.Biophys.Acta. 229:301-309 (1971) found that cleavage of peptidylthiohydantoins could be done in a repetitive manner with a protonated cation exchange resin. Application of this procedure to 100 .mu.mol quantities of papain and ribonuclease was reported to give 14 and 10 cycles, respectively, although no details were given. See Yamashita, S., et al., Proc.Hoshi.Pharm. 13:136-138 (1971). Stark reported that certain organic bases, such as morpholine or piperidine, could be substituted for sodium hydroxide, and along the same lines, Kubo, H., et al., Chem.Pharm.Bull. 19:210-211 (1971) reported that aqueous triethylamine (0.5M) could be used to effectively cleave peptidylthiohydantoins. Stark appeared to have solved the cleavage problem by introducing acetohydroxamic acid in aqueous pyridine at pH 8.2 as a cleavage reagent. This reagent was shown to rapidly and specifically cleave peptidylthiohydantoins at room temperature and at mild pH.
Conditions for the formulation of the peptidylthiohydantoins were improved by Stark and Dwulet, F. E., et al., Int.J.Peptide and Protein Res. 13:122-129 (1979), who reported on the use of thiocyanic acid rather than thiocyanate salts, and more recently by the introduction of trimethylsilylisothiocyanate (TMS-ITC) as a coupling reagent. See Hawke, D. H., et al., Anal.Biochem. 166:298-307 (1987). The use of this reagent for C-terminal sequencing has been patented. See Hawke U.S. Pat. No. 4,837,165. This reagent significantly improved the yields of peptidylthiohydantoin formation and reduced the number of complicating side products. Cleavage of peptidylthiohydantoins by 12 N HCl (Hawke, 1987) and by acetohydroxamate (Miller, C. G., et al., Techniques in Protein Chemistry (Hugli, T. E., ed.) pp. 67-68, Academic Press (1989)) failed to yield more than a few cycles of degradation.
B. The Cleavage Problem
Although the cleavage reaction has been extensively studied since the thiocyanate chemistry for C-terminal degradation was first proposed by Schlack and Kumpf in 1926, a chemical method has not yet been proposed that is capable of an extended degradation. Cleavage in 1N sodium hydroxide as first proposed by Schlack and Kumpf (1926) is well known to hydrolyze proteins and peptides at other sites in addition to cleavage of the C-terminal peptidylthiohydantoin. The released thiohydantoin amino acid derivatives are also known to be unstable in hydroxide solutions. Scoffone, supra. Cleavage by hydroxide is known to convert the side chain amide groups of asparagine and glutamine residues to a carboxylic group making these residues indistinguishable from aspartate and glutamate, respectively.
When cleavage of peptidylthiohydantoins by 12N HCl was applied to proteins and peptides no more than 2 or 3 cycles could be performed. See, Cromwell, supra and Hawke, supra. This was probably due to differences in the rate of hydrolysis of peptidylthiohydantoins containing different amino acid side chains as well as to hydrolysis of other internal amide bonds. Likewise, during the synthesis of the standard amino acid thiohydantoin derivatives corresponding to the naturally occurring amino acids, it was observed that the rate of deacetylation of the N-acetylthiohydantoin amino acids by 12 HCl depended on the nature of the amino acid side chain. Bailey, J. M., et al. Biochem. 29:3145-3156 (1990).
Attempts by Dwulet, supra, to reproduce the resin based cleavage method of Yamashita, supra, was reported to be unsuccessful. Cleavage of peptidylthiohydantoins with aqueous methanesulfonic acid was also attempted by Dwulet and by Bailey, et al., both without success. Methanesulfonic acid was chosen since it is equivalent to the acidic group on the resin employed by Yamashita (1971) and Yamashita, et al. (1971).
Cleavage of the peptidylthiohydantoin derivatives with acetohydroxamate as originally reported by Stark, supra, was found to result in the formation of stable hydroxamate esters at the C-terminus of the shortened peptide (Bailey, et al., supra). Depending on the conditions employed, between 68% and 93% of the peptide was derivatized at the C-terminus and thus prevented from further sequencing. Although Stark, supra, predicted such hydroxamate esters to form as an intermediate during cleavage, it was assumed that they would break down under the conditions used for cleavage or continued sequencing. The peptidyl hydroxamate esters formed from cleavage with acetohydroxamate, like the hydroxamate esters studied by Stieglitz, J., et al., J.Am.Chem.Soc. 36:272-301 (1914) and Scott, A. W., et al., J.Am.Chem.Soc. 49:2545-2549 (1927), are stable under the acidic conditions used for thiohydantoin formation and can only be hydrolyzed to a free peptidyl carboxylic group, capable of continued sequencing, under strongly basic conditions. This probably explains the low repetitive yields of Stark, supra; Meuth, J. L., et al., Biochem. 21:3750-3757 (1982) and Miller, supra, when aqueous acetohydroxamate was employed as a cleavage reagent.
Cleavage of peptidylthiohydantoins by aqueous triethylamine was originally reported by Kubo, H., et al., Chem.Pharm.Bull. 19:210-211 (1971), Dwulet, et al., supra, and Meuth, et al., supra. The latter group commented on the usefulness of triethylamine as a cleavage reagent for automated sequencing because of its volatility, but declined to pursue this method apparently in favor of cleavage by acetohydroxamate. Cleavage of peptidylthiohydantoins, in the solution phase, by a 2% aqueous solution of triethylamine was found to be rapid (half-times of 1 min. and 5 min. at 37.degree. C. and 22.degree. C., respectively) and quantitative, yielding only shortened peptide capable of continued sequencing and the amino acid thiohydantoin derivative. Bailey, et al., supra.